As the potential dangers of hazardous wastes in the environment have become better appreciated, regulatory and community pressure have made it necessary to return to sites of waste disposal or toxic contamination to reassess the possible hazardousness of wastes or contaminants disposed there and to take steps to reduce or eliminate dangers to people directly or to the surrounding environment in general and to ground water in particular.
Heavy metals such as lead, arsenic, cadmium, chromium, zinc and copper are common elements of concern in waste sites under investigation, with lead being the most commonly found. Leaching of heavy metals into ground water is of particular concern because of the danger that drinking water supplies would thus become contaminated. The U.S. EPA has designated maximum contaminant levels (MCL's) and secondary maximum contaminant levels (SMCL's) for heavy metals in the National Drinking Water Regulations presented in 40 CFR .sctn.141.11 and .sctn.143.3 (1990).
Solid wastes are classified as hazardous by the United States Environmental Protection Agency (U.S. EPA) for a number of reasons. Certain wastes are classified as hazardous because they contain chemicals which are listed by U.S. EPA as hazardous. Other wastes are classified as hazardous because of characteristics of the waste. These characteristics include ignitability, corrosivity, reactivity, and toxicity.
The characteristic of toxicity is determined using the Toxicity Characteristic Leaching Procedure Test (TCLP). The TCLP Test determines whether a solid waste has unacceptable levels of hazardous substances, such as heavy metals, which can be leached from the waste by infiltrating leachate. Wastes containing leachable lead and cadmium are currently classified as TCLP Toxic Waste if the level of cadmium extracted in a TCLP Toxicity test is above 1.0 mg/L or if the level of lead extracted is above 5.0 mg/L. Some states, notably Michigan, also classify wastes which leach high levels of copper and zinc as hazardous.
The TCLP Test is designed to simulate a worst-case leaching situation. These leaching conditions would typically be found in the interior of an actively degrading municipal landfill. In such landfills, the leaching medium is slightly acidic, with a pH of about 5. Additionally, the leaching medium is heavily buffered by volatile organic acids (e.g., acetic acid) produced by the facultative anerobic degradation of organic matter.
Tests are available which simulate the more typical disposal situation for hazardous wastes such as foundry waste. These tests utilize a relatively unbuffered solution, e.g., deionized water, to provide a better simulation of leaching as it actually occurs in the environment. Examples of nonacidic or not heavily buffered acidic leach tests, commonly referred to as water leach tests, include the Indiana Water Leach Test, which is also called the EP Water Leach Test (Indiana Administrative Code Title 329, Article 2, Solid Waste Management Rule 9); the U.S. EPA Multiple Extraction Procedure (U.S. EPA, 1986, updated 1991, Test Methods for Evaluating Solid Waste. Volume 1C, Method 1320 EPA/530/SW-846); the Synthetic Precipitation Leaching Procedure (U.S. EPA 1986, updated 1991, Test Methods for Evaluating Solid Wastes. Method 1312, EPA/530/SW-846); the American Society of Testing Materials Test (ATSM Standards, method D3987-85); the American Foundryman's Society Leach Test (Ham, R. K., W. C. Boyle and T. P. Kunes, J. Env. Eng. Div. Amer. Soc. Civil Eng., 107 EEL, pp. 155-170, 1981); and the University of Wisconsin SLT Test (Ham, R. K., M. A. Anderson, R. Stegmann and R. R. Stanforth, EPA 600/2-79/109, 1979).
For wastes that are not disposed of in acidic environments, two separate leach tests need to be run to determine whether the waste is hazardous according to regulatory standards, and to determine whether the waste could pose an actual environmental risk when exposed to nonacidic leachate in a disposal facility. The TCLP test will define the regulatory status of the waste. A water leach test will provide an indication of the actual leaching potential of the waste in the environment and indicate whether the Drinking Water Regulations (MCL's and SMCL's) are being exceeded.
U.S. Pat. Nos. 4,889,640 METHOD AND MIXTURE FOR TREATING HAZARDOUS WASTES; 4,950,409 METHOD FOR TREATING HAZARDOUS WASTES; and 5,037,479 METHOD FOR REDUCTION OF HEAVY METAL LEACHING FROM HAZARDOUS WASTE UNDER ACIDIC AND NON-ACIDIC CONDITIONS, are incorporated by reference herein and provide methods and mixtures for making wastes non-hazardous according to certain standards prior to deposit in a waste disposal site. Said patents disclose methods for chemically controlling the leaching process in waste.
U.S. Pat. No. 4,889,640 describes a method of treating solid hazardous waste containing unacceptable levels of leachable metals, such as lead and cadmium. It includes mixing the solid waste with an agent selected from the group consisting of reactive calcium carbonate, reactive magnesium carbonate, and reactive calcium magnesium carbonate. After the solid waste and agent are mixed under conditions which support reaction between the agent and metals, the metals will be converted into non-leachable forms which are relatively stable under normal environmental conditions. If the solid waste material and agent are both dry when mixed, it may be beneficial to add water to facilitate the mixing of the solid waste and agent and the conversion of the metals into non-leachable forms. The EP toxic wastes were treated and measured using the EP-toxicity test results to determine the amount of leaching of heavy metals after the treatment. The EP hazardous waste criteria was 1.0 mg per liter for cadmium and 5.0 mg per liter for lead.
U.S. Pat. No. 4,950,409 discloses a method of treating solid hazardous waste containing unacceptable levels of leachable metals such as lead and cadmium which includes mixing the solid waste with lime and an agent selected from the group of carbon dioxide and bicarbonate. After the solid waste and agent are mixed under conditions which support reaction between the agent and metals, the metals will be converted to non-leachable forms which are relatively stable under normal environmental conditions. If the solid waste and agent are both dry when mixed, it may be beneficial to add water to facilitate mixing of the solid waste and agent and conversion of the metals into non-leachable forms. The hazardous waste criterion for the EP toxicity test are 1.0 mg per liter cadmium and 5.0 mg per liter for lead.
U.S. Pat. No. 5,037,479 discloses a method of treating solid hazardous wastes containing unacceptable levels of leachable metals such as lead, cadmium and zinc which includes mixing the solid waste with a buffering agent selected from the group consisting of magnesium oxide, magnesium hydroxide, reactive calcium carbonate and reactive magnesium carbonate, and with an additional agent which is an acid or salt containing an anion that forms substantially non-leachable forms of the metals or is metallic iron that chemically reduces the metal to a non-leachable form. This anion agent is selected from the group consisting of triple superphosphate, phosphoric acid, or boric acid, so that under both acidic and non-acidic leaching conditions, the metals and mixture will be converted to substantially non-leachable forms. The criterion for the leachability uses the TCLP toxicity test whereby cadmium is extracted at a level below 1.0 mg per liter and lead is extracted at a level below 5.0 mg per liter, and also the water leach test whereby the criterion for cadmium is a level below 0.01 mg per liter and lead at a level below 0.05 mg per liter.
Common methods for treatment of contaminated waste sites usually involve excavating the contaminated material and treating it with a solidifying agent such as cement or some form of silicates. However, such treatment is costly and results in a large volume increase of the material. Such methods will sometimes employ a hollow-stem auger for injecting the cement or silicate material into the soil and then mixing in place. There is evidence to suggest, however, that such treatments are not always effective in preventing the leaching PG,7 of heavy metals such as lead. (See Immobilization Mechanisms n Solidification/Stabilization of Cd and Pb Salts Using Portland Cement Fixing Agents, Cartledge, et al. Environ. Sci. Technol. 1990, Vol. 24, No. 6, pp. 867-873.)
Another known method of remediating heavy metal contaminated sites is to wash the affected soil or waste with a solution and collect the leachate for removal and disposal. This method has drawbacks of being lengthy and expensive, and resulting in a collection of concentrated heavy metals which may themselves be hazardous wastes which must be carefully disposed of.
Methods which involve excavation and removal of waste material for treatment are costly, requiring excavation and trucking equipment and personnel. Furthermore, due to strict regulations for handling and transport of hazardous wastes and soil contaminated with hazardous waste, even temporarily removing loads of soil for treatment above ground and on site may require specific permits or variances. For this reason, a treatment method which renders waste non-hazardous without any excavation is highly desirable.
What is needed is a method for treating waste sites containing heavy metals which chemically controls the leaching of heavy metals and which may economically be employed without the need for excavation of the site.